Liquid crystal display device and driving method thereof

ABSTRACT

Disclosed is a liquid crystal display device and a driving method thereof for displaying an image, in which the polarity of a voltage applied to the liquid crystal element is inverted in a first frame period and a second frame period which are sequential. The voltage applied to the liquid crystal element is compensated in the case where images of the first frame period and the second frame period are judged as a still image as a result of comparison of the image of the first frame period with the image of the second frame period and the absolute value of the voltage applied to the liquid crystal element in the first frame period is different from that of the voltage applied to the liquid crystal element in the second frame period.

TECHNICAL FIELD

The present invention relates to a liquid crystal display device and amethod for driving the liquid crystal display device.

BACKGROUND ART

Liquid crystal display devices have been applied in a wide range offields from large display devices such as TV sets to small displaydevices such as mobile phones, and those with higher added value havebeen developed. In recent years, in view of rising interest in theglobal environment and improvement of the convenience of mobile devices,development of liquid crystal display devices with low power consumptionhas attracted attention.

For example, Patent Document 1 discloses a technique for reducing thepower consumption of a liquid crystal display device, according to whichall of the signal lines are electrically disconnected to a signal linedriver circuit to have high impedance in order to keep the voltage ofeach signal line constant in a break period during which no scan lineand no signal line are selected.

Non-Patent Document 1 discloses a structure for reducing the powerconsumption of a liquid crystal display device, according to which therefresh rate is made to be different in the case of moving image displayand in the case of still image display. Non-Patent Document 1 furtherdiscloses a technique for preventing fluctuation of a voltage applied toa liquid crystal element in order to prevent a flicker due to thefluctuation of the voltage applied to the liquid crystal element, whichis accompanied by switching of a signal between a break period and ascan period in the case where a still image is displayed, according towhich AC signals with the same phase are applied to a signal line and acommon electrode also in the break period.

REFERENCE

-   Patent Document 1: Japanese Published Patent Application No.    2001-312253-   Non-Patent Document 1: Kazuhiko Tsuda et al., IDW'02, pp 295-298

DISCLOSURE OF INVENTION

However, in the above-referenced driving methods, a configuration of adriver circuit which supplies a signal to a scan line or a signal lineof the liquid crystal display device and an operation thereof arecomplicated, so that power consumption of the liquid crystal displaydevice cannot be reduced enough.

Further, in the case where the break period during which no scan lineand no signal line are selected is provided as described in PatentDocument 1, the interval between writing operations into pixels becomeslong, by which the voltage applied to a liquid crystal element islargely decreased by leakage current of a transistor or parasiticcapacitance in some cases. Large decrease in the voltage applied to theliquid crystal element lowers display quality; for example, display witha predetermined gray scale level cannot be performed.

For example, in liquid crystal display devices, in order to suppressoccurrence of image burn-in, a driving method in which the levels(polarities) of the voltages applied to a pair of electrodes of a liquidcrystal element are inverted per frame period (the driving method alsoreferred to as inversion driving) has been used.

In the case where the inversion driving is performed, decrease of thevoltage applied to the liquid crystal element due to the leakage currentof the transistor causes a difference of the gray scale level of imagesdisplayed in respective two sequential frame periods even when imagesignals for the same image are input to the two sequential frameperiods.

In view of the above, an object of one embodiment of the presentinvention is to provide a liquid crystal display device which does notneed a complicated operation in a driver circuit of the liquid crystaldisplay device and consumes less power consumption in the case where astill image is displayed. Further, an object of one embodiment of thepresent invention is to suppress deterioration of the display quality.

One embodiment of the present invention is a liquid crystal displaydevice in which a transistor including an oxide semiconductor layerfunctioning as a channel formation layer is provided in a pixel, inwhich a voltage applied to a liquid crystal element is compensated indisplaying a still image so that fluctuation of the gray scale level ofa still image between subsequent frame periods is suppressed.

One embodiment of the present invention is a driving method of a liquidcrystal display device including a pixel and a driver circuit forcontrolling whether or not an image signal is supplied to the pixel. Thepixel includes a liquid crystal element including a first terminal towhich the image signal is supplied and a second terminal to which acommon voltage is input, and a transistor for controlling whether or notthe image signal is supplied to the first terminal of the liquid crystalelement. The transistor includes an oxide semiconductor layerfunctioning as a channel formation layer. A polarity of a voltage whichis applied to the liquid crystal element is inverted between a firstframe period and a second frame period which are sequential frameperiods, so that image display is performed. According to the drivingmethod, the voltage applied to the liquid crystal element is compensatedin the case where an image formed by the images in the first frameperiod and the second frame period are judged as a still image as aresult of comparison of the image in the first frame period with theimage in the second frame period and the absolute value of the voltageapplied to the liquid crystal element in the first frame period isdifferent from that of the voltage applied to the liquid crystal elementin the second frame period.

One embodiment of the present invention is a driving method of a liquidcrystal display device including a pixel and a driver circuit forcontrolling whether or not an image signal is supplied to the pixel. Thepixel includes a liquid crystal element including a first terminal towhich the image signal is supplied and a second terminal to which acommon voltage is input and a transistor for controlling whether or notthe image signal is supplied to the first terminal of the liquid crystalelement. The transistor includes an oxide semiconductor layerfunctioning as a channel formation layer and having a carrierconcentration of less than 1×10¹⁴/cm³. A polarity of the voltage whichis applied to the liquid crystal element is inverted between a firstframe period and a second frame period which are sequential frameperiods, so that image display is performed. According to the drivingmethod, the voltage applied to the liquid crystal element is compensatedin the case where an image formed by the images of the first frameperiod and the second frame period are judged as a still image as aresult of comparison of the image of the first frame period with theimage of the second frame period and the absolute value of the voltageapplied to the liquid crystal element in the first frame period isdifferent from that of the voltage applied to the liquid crystal elementin the second frame period.

According to one embodiment of the present invention, power consumptionin the case where a still image is displayed on a liquid crystal displaydevice can be reduced. Further, according to one embodiment of thepresent invention, deterioration of the display quality can besuppressed.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 illustrates a structure example of a liquid crystal displaydevice in Embodiment 1;

FIG. 2 illustrates an example of a pixel structure of a liquid crystaldisplay device in Embodiment 1;

FIG. 3 illustrates an operation example of a liquid crystal displaydevice in Embodiment 1;

FIG. 4 illustrates an operation example of a liquid crystal displaydevice in Embodiment 1;

FIGS. 5A to 5C illustrate a structure example of a driver circuit inEmbodiment 2;

FIG. 6 illustrates an operation example of a driver circuit inEmbodiment 2;

FIG. 7 illustrates an operation example of a driver circuit inEmbodiment 2;

FIGS. 8A to 8D illustrate a transistor in Embodiment 3;

FIGS. 9A to 9C illustrate examples of a liquid crystal display device inEmbodiment 4;

FIGS. 10A and 10B illustrate structure examples of a liquid crystaldisplay device in Embodiment 5;

FIG. 11 illustrates a structure example of an e-book reader inEmbodiment 6; and

FIGS. 12A to 12F illustrate structure examples of electronic devices inEmbodiment 7.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention and examples thereof will behereinafter described with reference to the accompanying drawings. Thepresent invention can be carried out in many different modes, and it iseasily understood by those skilled in the art that modes and details ofthe present invention can be modified in various ways without departingfrom the purpose and the scope of the present invention. Accordingly,the present invention is not construed as being limited to the describedcontent of the embodiments and examples included herein. Note thatidentical portions or portions having the same function are denoted bythe same reference numerals throughout the drawings in the structures ofthe present invention described below.

The size, the layer thickness, or the area of each structure isexaggerated in the drawings of the embodiments for clarification in somecases; therefore, the embodiments of the present invention are notlimited to such scales.

Numerals such as an “N-th” (N is a natural number) are used for avoidingconfusion of components in this specification and do not imply thenumber of the components.

(Embodiment 1)

In Embodiment 1, a driving method of a liquid crystal display deviceaccording to one embodiment of the present invention and the liquidcrystal display device will be described.

First, a structure of the liquid crystal display device in thisembodiment is described with reference to FIG. 1. FIG. 1 is a blockdiagram illustrating an example of the structure of the liquid crystaldisplay device in this embodiment.

A liquid crystal display device shown in FIG. 1 includes a display panel101, a memory circuit 103, a comparison circuit 104, a selection circuit105, and a display control circuit 106.

The display panel 101 includes a driver circuit portion 107 and a pixelportion 108.

The driver circuit portion 107 includes a driver circuit 109A and adriver circuit 109B. The pixel portion 108 includes a plurality ofpixels. The driver circuits 109A and 109B are driver circuits fordriving the plurality of pixels in the pixel portion 108. The drivercircuit 109A functions as a scan line driver circuit for controlling ascan line for selecting a pixel into which image data is written. Thedriver circuit 109B is a driver circuit which controls whether or not animage signal is supplied to a pixel and functions as a signal linedriver circuit for controlling a signal line to which an image signalincluding image data is supplied. In the liquid crystal display deviceof this embodiment, the driver circuits 109A and 109B can includetransistors.

The memory circuit 103 is a circuit into which an image signal (alsoreferred to as a signal Data) is input and which holds data of the imagesignal (also referred to as image data) for a certain period. The memorycircuit 103 includes a frame memory 110. The frame memory 110 storesdata of image signals of a plurality of frames. The number of the framememories 110 included in the memory circuit 103 is not particularlylimited; as shown in FIG. 1, the memory circuit 103 may include aplurality of the frame memories 110. In the liquid crystal displaydevice of this embodiment, the frame memory 110 may include a memoryelement such as a dynamic random access memory (DRAM) or a static randomaccess memory (SRAM).

The comparison circuit 104 is a circuit which selectively reads out dataof image signals in subsequent frame periods stored in the memorycircuit 103, compares the data of the image signals, and detects adifference thereof. For example, data of the image signals of first ton-th frame periods are stored (n is a natural number larger than 1), andthe comparison circuit 104 compares data of an image signal of an m-thframe period with data of an image signal of an (m+1)-th frame period (mis a natural number smaller than n). In the comparison circuit 104, thedifference is detected, whereby whether the data of the image signals inthe subsequent frame periods are data of image signals for displaying amoving image or data of image signals for displaying a still image isjudged. In the liquid crystal display device of this embodiment, thedetection criterion of a difference may be set such that the differencecan be recognized when the difference detected by the comparison circuit104 exceeds a certain value.

In this specification, the term “moving image” means an image which isrecognized as being changed in sequential frame periods when a pluralityof images temporally divided into the images in a plurality of framesare switched for operation. The term “still image” means an image whichis recognized as not being changed in sequential frame periods when aplurality of images temporally divided into the images in a plurality offrames are switched for operation.

The selection circuit 105 is a circuit which selects, when data of imagesignals compared in the comparison circuit 104 is judged as data ofimage signals for displaying a moving image (i.e., when the data of theimage signal of the m-th frame period is judged as being different fromthe data of the image signal of the (m+1)-th frame period), the data ofthe image signals from the frame memory (memories) 110 and outputs thedata of the image signals as image signals to the display controlcircuit 106. The selection circuit 105 includes a circuit including aplurality of switches such as transistors. In the case where nodifference of data of image signals between sequential frame periods isdetected in the comparison circuit 104 (i.e., the data of the imagesignal of the m-th frame period is judged as being the same as the dataof the image signal of the (m+1)-th frame period), images displayed inthe frame periods are a still image. In that case, the data of the imagesignal of the latter frame period is not output as an image signal tothe display control circuit 106 in this embodiment.

The display control circuit 106 is a circuit to which an image signal, astart signal (also referred to as a start pulse or a signal SP), a clocksignal (also referred to as a signal CK), a reset signal (also referredto as a signal Res), a high power source voltage (also referred to as avoltage Vdd), and a low power source voltage (also referred to as avoltage Vss) are supplied and which controls whether or not one or moreof the signals and voltages is supplied to the driver circuit portion107. For example, in the case where images in sequential frame periodsare judged as a moving image by the comparison of the comparison circuit104, image signals are supplied from the selection circuit 105 to thedisplay control circuit 106, and the start signal SP, the clock signalCK, the reset signal Res, the high power source voltage Vdd, and the lowpower source voltage Vss are supplied to the driver circuit portion 107.On the other hand, in the case where an image formed by images ofsequential frame periods is judged as a still mage by the comparison ofthe comparison circuit 104, an image signal of the latter frame periodis not supplied from the selection circuit 105 and one or more of thestart signal SP, the clock signal CK, the reset signal Res, the highpower source voltage Vdd, and the low power source voltage Vss is notsupplied to the driver circuit portion 107, in some cases.

Note that the term “voltage” generally means a difference betweenpotentials at two points (also referred to as a potential difference).However, values of both a voltage and a potential are represented usingvolt (V) in a circuit diagram or the like in some cases, so that it isdifficult to discriminate between them. Thus, in this specification, apotential difference between a potential at one point and a referencepotential is sometimes used as a voltage at the point unless otherwisespecified.

Further, a transmissive type liquid crystal display device, asemi-transmissive type liquid crystal display device, or a reflectivetype liquid crystal display device can be employed as the liquid crystaldisplay device of this embodiment.

A progressive type display, an interlace type display, or the like canbe employed as the display type of the liquid crystal display device ofthis embodiment. Further, color elements controlled in a pixel at thetime of color display are not limited to three colors: R, G, and B (R,G, and B correspond to red, greed, and blue, respectively). For example,R, G, B, and W (W corresponds to white); R, G, B, and one or more ofyellow, cyan, magenta, and the like; or the like can be used. Further,the size of the display region may be different depending on a dot of acolor element. The liquid crystal display device of this embodiment isnot limited to a display device for color display; this embodiment canalso be applied to a display device for monochrome display.

Next, a circuit configuration of the pixel in FIG. 1 will be describedwith reference to FIG. 2. FIG. 2 is an equivalent circuit diagramillustrating a structural example of the pixel included in the pixelportion 108 in FIG. 1.

A pixel shown in FIG. 2 includes a transistor 151, a liquid crystalelement 152, and a capacitor 153.

In this specification, the transistor has at least a source, a drain,and a gate. As the transistor, a gate-insulated transistor can be used,for example.

The gate means the whole of a gate electrode and a gate wiring or partthereof. A conductive layer having a function of both a gate electrodeand a gate wiring is referred to as a gate in some cases withoutdistinction between the gate electrode and the gate wiring.

The source means the whole of a source electrode and a source wiring orpart thereof. A conductive layer having a function of both a sourceelectrode and a source wiring is referred to as a source in some caseswithout distinction between the source electrode and the source wiring.

The drain means the whole of a drain electrode and a drain wiring orpart thereof. A conductive layer having a function of both a drainelectrode and a drain wiring is referred to as a drain in some caseswithout distinction between the drain electrode and the drain wiring.

Further, in this specification, a source and a drain of a transistor mayinterchange with each other depending on the structure, the operatingcondition, and the like of the transistor; therefore, it is difficult tofix the source and the drain. Therefore, in this document (thespecification, the claims, the drawings, or the like), one of them isreferred to as one of the source and the drain, and the other isreferred to as the other of the source and the drain.

One of a source and a drain of the transistor 151 is electricallyconnected to the signal line 154 and the gate thereof is electricallyconnected to a scan line 155.

The liquid crystal element 152 has a first terminal and a secondterminal. The first terminal is electrically connected to the other ofthe source and the drain of the transistor 151, and the second terminalis electrically connected to a wiring 156. The liquid crystal element152 can include a first electrode which serves as part of or the wholeof the first terminal, a second electrode which serves as part of or thewhole of the second terminal, and a liquid crystal layer whosetransmittance of light is changed by application of voltage between thefirst electrode and the second electrode.

The specific resistance of a liquid crystal material of the liquidcrystal element 152 is 1×10¹² Ω·cm or more, preferably 1×10¹³ Ω·cm ormore, far preferably 1×10¹⁴ Ω·cm or more. The specific resistance inthis specification is defined as that measured at 20° C. In the casewhere a liquid crystal display device is formed using the liquid crystalmaterial, the resistance of a portion serving as a liquid crystalelement may be 1×10¹¹ Ω·cm or more, furthermore 1×10¹² Ω·cm or more insome cases because of an impurity mixed into a liquid crystal layer froman alignment film, a sealant, or the like.

The larger the specific resistance of the liquid crystal material is,the more the leakage current of the liquid crystal layer can besuppressed and the more the decrease over time of the voltage applied tothe liquid crystal element in the display period can be suppressed. As aresult, the display period can be extended, so that the frequency ofsignal writing can be decreased, which leads to reduction of powerconsumption of the liquid crystal display device.

The capacitor 153 has a first terminal and a second terminal. The firstterminal is electrically connected to the other of the source and thedrain of the transistor 151, and the second terminal is electricallyconnected to a wiring 157. The capacitor 153 serves as a storagecapacitor and can include a first electrode which serves as part of orthe whole of the first terminal, a second electrode which serves as partof or the whole of the second terminal, and a dielectric layer in whichelectric charge is accumulated by application of voltage between thefirst electrode and the second electrode. The capacitance of thecapacitor 153 may be set in consideration of the off-state current ofthe transistor 151. In this embodiment, a transistor including ahigh-purity oxide semiconductor layer is used as the transistor 151, andtherefore, a storage capacitor having capacitance which is less than orequal to ⅓, preferably less than or equal to ⅕ with respect to theliquid crystal capacitance of each pixel is sufficient to be provided.The capacitor 153 is not necessarily provided. Omission of the capacitor153 can improve the aperture ratio of the pixel.

The wiring 156 serves as a voltage line to which a certain voltage isapplied. For example, a common voltage (also referred to as a voltageV_(COM)) is applied to the wiring 156. The common voltage may be apositive voltage, a negative voltage, or ground potential.

The wiring 157 serves as a voltage line to which a certain voltage isapplied. For example, a unit voltage is applied to the wiring 157. Theunit voltage may be a common voltage.

Further, a switch may be provided between the second terminal of theliquid crystal element 152 and the wiring 156, and the switch may beturned on so that a common voltage is applied to the second terminal ofthe liquid crystal element 152 in a writing period and the switch may beturned off so that the second terminal of the liquid crystal element 152has a floating state in a display period. It is preferable to use atransistor applicable to the transistor 151, as the switch. Accordingly,the voltage applied to the liquid crystal element 152 can be preventedfrom being fluctuated at the time of displaying a still image.

A switch may be provided between the second terminal of the capacitor153 and the wiring 157, and the switch may be turned on so that a unitvoltage is applied to the second terminal of the capacitor 153 in awriting period and the switch may be turned off so that the secondterminal of the capacitor 153 has a floating state in a display period.It is preferable to use a transistor applicable to the transistor 151,as the switch. Accordingly, the voltage applied to the capacitor 153 canbe prevented from being fluctuated at the time of displaying a stillimage. The above description is made on the configuration of the pixelshown in FIG. 2.

Next, a transistor applicable to the transistor included in the drivercircuit 109A or the driver circuit 109B or the transistor 151 isdescribed below.

As the transistor included in the driver circuit 109A or the drivercircuit 109B or the transistor 151, a transistor including an oxidesemiconductor layer functioning as a channel formation layer can beused, for example. The oxide semiconductor layer functioning as achannel formation layer of the transistor is an intrinsic (i-type) or asubstantially intrinsic oxide semiconductor which is obtained byremoving hydrogen that is an n-type impurity from an oxide semiconductorand highly purifying the oxide semiconductor such that an impurity otherthan a main component of the oxide semiconductor is not contained asmuch as possible. In other words, the oxide semiconductor layer has afeature in that it is made to be an i-type (intrinsic) semiconductor ormade to be close thereto not by addition of an impurity but by thoroughpurification to remove an impurity such as hydrogen or water as much aspossible.

As the oxide semiconductor, any of the following can be used: an oxideof four metal elements such as In—Sn—Ga—Zn—O; an oxide of three metalelements such as In—Ga—Zn—O, In—Sn—Zn—O, In—Al—Zn—O, Sn—Ga—Zn—O,Al—Ga—Zn—O, or Sn—Al—Zn—O; an oxide of two metal elements such asIn—Zn—O, Sn—Zn—O, Al—Zn—O, Zn—Mg—O, Sn—Mg—O, In—Mg—O, or In—Sn—O; In—O;Sn—O; and Zn—O. Further, SiO₂ may be included in the oxidesemiconductor.

As the oxide semiconductor, a material represented by InMO₃(ZnO)_(m)(m>0) can be used. Here, M represents one or more metal elementsselected from Ga, Al, Mn, and Co. For example, M may be Ga, Ga and Al,Ga and Mn, Ga and Co, or the like. Among the oxide semiconductorsrepresented by InMO₃(ZnO)_(m) (m>0), an oxide semiconductor whichincludes Ga as M is referred to as an In—Ga—Zn—O oxide semiconductor.

Furthermore, the bandgap of the oxide semiconductor used as the oxidesemiconductor layer is 2 eV or more, preferably 2.5 eV or more, farpreferably 3 eV or more. Accordingly, the number of carriers generatedby thermal excitation can be reduced to a negligible number. Further,the amount of impurity such as hydrogen which becomes a donor is reducedto a certain amount or less so that the carrier concentration is lessthan 1×10¹⁴/cm³, preferably 1×10¹²/cm³ or less. That is, the carrierconcentration of the oxide semiconductor layer is reduced to as close tozero as possible.

The purification is carried out under at least one of the followingconcepts: removal of hydrogen from an oxide semiconductor layer as muchas possible; and reduction of defects, which are caused by oxygendeficiency in an oxide semiconductor layer, by supply of oxygen to theoxide semiconductor layer.

In the above-described transistor including the oxide semiconductorlayer, the off-state current per channel width of 1 μm can be reduced to10 aA/μm (1×10⁻¹⁷ A/μm) or less, further to 1 aA/μm (1×10⁻¹⁸ A/μm) orless, still further to 10 zA/μm (1×10⁻²⁰ A/μm) or less.

In the case where a liquid crystal display device is formed using thetransistor off-state current of which is extremely small as describedabove, the display period of an image per writing of image data can belong because the leakage current due to the transistor is extremelysmall. Therefore, the interval between writings of mage data can beprolonged. Further, the frame frequency can be lowered. For example, thewriting interval of image data can be prolonged to 10 seconds or longer,preferably 30 seconds or longer, far preferably 1 minute or longer. Inaddition, power consumption at the time of displaying a still image canbe reduced. As the interval between writings of image data is prolonged,power consumption can be reduced.

Further, in the above-described transistor including the oxidesemiconductor layer, fluctuation of the electrical characteristicsdepending on the temperature is small; for example, the dependency ofthe on-state current or the off-state current of the transistor on thetemperature in the range from −30° C. to 120° C. can be considered to bezero.

Next, a driving method of the liquid crystal display device shown inFIG. 1 including the pixel shown in FIG. 2 is described below.

In the driving method of the liquid crystal display device shown in FIG.1, data of an image signal is input to the memory circuit 103.

The memory circuit 103 holds data of image signals of sequential frameperiods, and outputs the data of the image signals to the comparisoncircuit 104 as image signals.

The comparison circuit 104 compares input data of image signals of thesequential frame periods (e.g., a first frame period and a second frameperiod) with each other and detects a difference thereof, therebyjudging whether an image is a moving image or a still image on the basisof the compared data of the image signals. When a difference isdetected, the image is judged to be a moving image; when no differenceis detected, the image is judged to be a still image.

In the case where images are judged to be a moving image on the basis ofthe comparison of data of the image signals, the selection circuit 105outputs the data of the image signals held in the memory circuit 103 asimage signals to the display control circuit 106. Furthermore, in thatcase, the display control circuit 106 supplies data of the input imagesignals as image signals to the driver circuit portion 107.

On the other hand, in the case where images are judged to be a stillimage on the basis of the comparison of data of the image signals, asupply of the image signal for the latter frame period (e.g., the secondframe period) to the display control circuit 106 is stopped.Furthermore, in that case, a supply of the image signal for the latterframe period (e.g., the second frame period) to the driver circuitportion 107 is stopped. Further, in addition to the image signal, thesupply of one or more of the clock signal CK, the start signal SP, thereset signal Res, the high power source voltage Vdd, and the low powersource voltage Vss to the driver circuit portion 107 can be stopped. Bystopping the supply of the above-described signal(s) and/or voltage(s),the driver circuit portion can be stopped operating during the period inwhich the still image is displayed.

Further, in the case where data of the image signal in the first frameperiod is the same as data of an image signal in the preceding frameperiod, i.e., images displayed in the two sequential frame periods are astill image, a supply of the image signal to the driver circuit portion107 can be stopped and a supply of the image signal from the drivercircuit portion 107 to the pixel portion 108 can be stopped in the firstframe period. Further, in that case, the supply of one or more of theclock signal CK, the start signal SP, the reset signal Res, the highpower source voltage Vdd, and the low power source voltage Vss to thedriver circuit portion 107 can be stopped, whereby power consumption canbe reduced.

As described above, according to one example of the driving method ofthe liquid crystal display device in this embodiment, data of aplurality of image signals is compared with each other, so that whetherimages to be displayed are a moving image or a still image is judged,and the supply of a control signal such as a clock signal or a startsignal to the driver circuit portion is selectively restarted orstopped. By selectively performing the supply of a signal or a voltageto the driver circuit portion, a period during which the driver circuitportion is stopped operating can be provided, which leads to reductionof power consumption of the liquid crystal display device.

Moreover, in the liquid crystal display device of this embodiment, sincethe transistor using an oxide semiconductor whose off-state current issmaller than that of an amorphous silicon TFT as descried above, adisplay period of an image by one image-data writing can be long.

Further, an example of a writing operation and a display operation of apixel is described below with reference to FIG. 3. FIG. 3 is a timingchart for describing an example of a writing operation and a displayoperation of the pixel of this embodiment.

For the pixel, a writing period 211 and a display period (also referredto as a holding period) 212 are provided in each of a first frame period201 and a second frame period 202.

In the writing period 211, as shown in FIG. 3, a scan signal which isinput through a scan line is active (the scan signal (also referred toas V_(G)) is at high level in FIG. 3). Then, the transistor 151 in thepixel is turned on, and a voltage of an image signal is supplied to thefirst terminal of the liquid crystal element 152 and the first terminalof the capacitor 153 via the transistor 151, whereby image data iswritten. In the display period 212, the pixel keeps a display statecorresponding to the written image data.

Furthermore, in the liquid crystal display device of this embodiment, inorder to suppress image burn-in on the display panel 101, a drivingmethod is employed in which the supply of an image signal to the drivercircuit portion 107 is restarted and the polarity of the voltage appliedto the liquid crystal element 152 is inverted in the case where stillimage is displayed in sequential frame periods the number of whichexceeds a predetermined number (the driving method also referred to as aframe inversion driving). Namely, in the case where still image isdisplayed using a plurality of frame periods that can be regarded asbeing the same image, the polarity inversion of the voltage applied tothe liquid crystal element 152 is conducted only when the number of thesequential frame periods exceed a standard value. For example, when astill image is displayed using first to (n+1)-th frame periods (n is anatural number larger than 1), after the data of the image signal of thefirst frame period is supplied to the liquid crystal element 152 throughthe driver circuit portion 107, the supply of the data is stopped in thesequential second to n-th frame periods. Then, the inversion of thevoltage is carried out between the n-th and (n+1)-th frame periods. Notethat the polarity inversion of the voltage may be conducted using acommon voltage as a standard. The driving method of the liquid crystaldisplay device of this embodiment is not limited to this example;another driving method such as a scan-line inversion driving, asignal-line inversion driving, a dot inversion driving, or acommon-voltage inversion driving may be employed.

The supply of the image signal is restarted at a timing at which thenumber of frame periods during which one still image is displayedcontinuously exceeds a predetermined number. The number of frame periodscan be counted by a counting circuit provided, for example. In thatcase, the supply of the image signal to the driver circuit portion 107is restarted when the counting value of the counting circuit exceeds apredetermined value, and the polarity of the voltage applied to theliquid crystal element 152 is inverted. In the timing chart shown inFIG. 3, the counting value exceeds the predetermined value in the secondframe period 202, the supply of the image signal to the driver circuitportion 107 is restarted, and the polarity of the voltage applied to theliquid crystal element 152 is inverted. In addition, in the case wherethe supply of one or more of the clock signal CK, the start signal SP,the reset signal Res, the high power source voltage Vdd, and the lowpower source voltage Vss to the driver circuit portion 107 is stopped atthe same time as the time at which the supply of the image signal to thedriver circuit portion 107 is stopped, the supply of one or more of theclock signal CK, the start signal SP, the reset signal Res, the highpower source voltage Vdd, and the low power source voltage Vss to thedriver circuit portion 107 may be restarted.

In the case where data of respective image signals in sequential twoframe periods (e.g., a first frame period and a second frame period) arecompared with each other using the image signals input, and an image inthe second frame period is judged to be the same as an image in thefirst frame period, it is preferable that absolute values of thevoltages applied to the liquid crystal element 152 in the sequentialframe periods be equal to each other.

However, as shown in FIG. 3, in some cases, the voltage applied to theliquid crystal element 152 (the voltage also referred to as a voltageV_(LC)) changes from (V11−V_(COM)) to (V12−V_(COM)) in the first frameperiod 201 and the voltage applied to the liquid crystal element 152changes from (V13−V_(COM)) to (V14−V_(COM)) in the second frame period202, which may result in a difference between an absolute value of adifference between the voltage (data) of an image signal in the firstframe period 201 and a common voltage (the absolute value is an absolutevalue of the voltage applied to the liquid crystal element 152 in thefirst frame period 201) and an absolute value of a difference betweenthe voltage (data) of an image signal in the second frame period 202 andthe common voltage (the absolute value is an absolute value of thevoltage applied to the liquid crystal element 152 in the second frameperiod 202). Such a voltage fluctuation is caused by feed-through or thelike, and causes deterioration of the display quality (e.g., generationof a flicker). The longer the display period by one image-data writingis, the more the voltage fluctuation may affect.

In view of the above, compensation is performed in an example of thedriving method of the liquid crystal display device of this embodiment.In the case where the absolute value of the voltage applied to theliquid crystal element 152 in the first frame period 201 is differentfrom that of the voltage applied to the liquid crystal element 152 inthe second frame period 202, the voltage applied to the liquid crystalelement 152 is compensated in the first frame period 201 or the secondframe period 202. In that case, the compensation is preferably performedsuch that the gray scale level of the pixel at the voltage applied tothe liquid crystal element 152 in the first frame period 201 is equal tothat of the pixel at the voltage applied to the liquid crystal element152 in the second frame period 202. For example, the voltage V_(COM) maybe compensated, so that the voltage applied to the liquid crystalelement 152 can be compensated, whereby the gray scale level can becompensated. The voltage applied to the capacitor 153 may becompensated. For example, the unit voltage applied to the secondterminal of the capacitor 153 is compensated, thereby compensating thevoltage applied to the capacitor 153.

The gray scale level of the pixel at the voltage applied to the liquidcrystal element 152 can be represented on the vertical axis of a graphwith the voltage applied to the liquid crystal element (also referred toas a voltage V_(LC)) represented on the horizontal axis, for example.For example, in the case where TN liquid crystal is used as a liquidcrystal material, the gray scale level at a positive voltage V_(LC) andthe gray scale level at a negative voltage V_(LC) can be expressed by astraight line 231 and a straight line 232 in FIG. 4, respectively. Asthe gray scale level increases, the image gets closer to white display;as the gray scale level decreases, the image gets closer to blackdisplay.

For example, in the case where the absolute value of the voltage appliedto the liquid crystal element 152 in the first frame period 201 issmaller than that of the voltage applied to the liquid crystal element152 in the second frame period 202, the voltage V_(COM) is shifted to bedecreased, so that the voltage applied to the liquid crystal element 152can be lowered, whereby the gray scale level of the pixel at the voltageapplied to the liquid crystal element 152 in the first frame period 201and that of the pixel at the voltage applied to the liquid crystalelement 152 in the second frame period 202 can be the same or as closeto each other as possible. When a still image is displayed using firstto (n+1)-th frame periods (n is a natural number larger than 1), forexample, the inversion of the voltage is carried out between the n-thand (n+1)-th frame periods, and the compensation is performed so thatthe voltage applied to the liquid crystal element 152 in the n-th frameperiod is the same as that in the (n+1)th frame period.

The compensation may be performed before the image signal supply to thedriver circuit portion after an image formed by the images of thesequential frame periods are judged to be a still image by thecomparison circuit 104. In that case, a compensation circuit isprovided, the absolute value of the voltage applied to the liquidcrystal element 152 in the first frame period 201 is compared with thatof the voltage applied to the liquid crystal element 152 in the secondframe period 202 in the comparison circuit 104, comparison data isoutput to the comparison circuit, and the voltage applied to the liquidcrystal element 152 is compensated by the compensation circuit inaccordance with the compensation data.

The voltage applied to the liquid crystal element 152 may be compensatedwhile comparing respective images displayed on the display panel in thefirst frame period 201 and the second frame period 202. The framefrequency may be lowered to about 1/10 of the normal one at the time ofthe compensation and returned to the normal one at the time of display.Lowering of the frame frequency enables more accurate compensation.

As described above, according to one example of the driving method ofthe liquid crystal display device of this embodiment, in the case whererespective images in the first frame period and the second frame periodare compared with each other, so that the image formed by the images ofthe first and second frame periods is judged to be a still image withrespect to the image of the first frame period, and the absolute valueof the voltage applied to the liquid crystal element in the first frameperiod is different from that of the voltage applied to the liquidcrystal element in the second frame period, the voltage applied to theliquid crystal element is compensated in the first frame period or thesecond frame period. Accordingly, even in the case where a still imageis displayed for a long time by applying to the liquid crystal element avoltage whose polarity is inverted in a plurality of sequential frameperiods, deterioration of the display quality due to voltage fluctuationcan be suppressed. For example, generation of a flicker can besuppressed. Therefore, in the case where a transistor including an oxidesemiconductor layer functioning as a channel formation layer is used asin the liquid crystal display device of this embodiment and a stillimage is displayed for a long period by one image-data writing, thedisplay quality can be prevented from deteriorating and powerconsumption can be reduced by the driving method of the liquid crystaldisplay device of this embodiment.

(Embodiment 2)

In Embodiment 2, a structure of a shift register included in each of ascan line driver circuit and a signal line driver circuit of the liquidcrystal display device described in Embodiment 1 will be described.

An example of the structure of the shift register in this embodiment isdescribed with reference to FIGS. 5A to 5C. FIGS. 5A to 5C are diagramsillustrating an example of the structure of the shift register in thisembodiment.

A shift register shown in FIG. 5A includes first to N-th pulse outputcircuits 10_1 to 10_N (N is a natural number greater than or equal to3).

Each of the first to N-th pulse output circuits 10_1 to 10_N iselectrically connected to three wirings among first to fourth wirings 11to 14. In the shift register shown in FIG. 5A, a first clock signal CK1is supplied through the first wiring 11, a second clock signal CK2 issupplied through the second wiring 12, a third clock signal CK3 issupplied through the third wiring 13, and a fourth clock signal CK4 issupplied through the fourth wiring 14.

A start signal SP1 is input to the first pulse output circuit 10_1through a fifth wiring 15.

A signal from the pulse output circuit 10_(n−1) (n is a natural numberof 2 or more and N or less) in the previous stage (the signal referredto as a former-stage signal OUT(n−1)) is input to the n-th pulse outputcircuit 10 _(—) n in a second or subsequent stage. Further, a signalfrom a third-pulse output circuit 10_3 in the third stage is input tothe first pulse output circuit 10_1 in the first stage; similarly, asignal from the (l+2)-th pulse output circuit 10_(l+2) (l is a naturalnumber of 2 or more and N−2 or less) in the (l+2)-th stage (the signalreferred to as a later-stage signal OUT(l+2)) is input to the l-th pulseoutput circuit 10 _(—) l in the l-th stage. Further, each pulse outputcircuit in each stage outputs a first output signal and a second outputsignal. As shown in FIG. 5A, the later-stage signals OUT(l+2) are notinput to the pulse output circuits in the (N−1)-th stage and the N-thstage; therefore, for example, a start signal SP2 may be input to thepulse output circuit in the (N−1)-th stage through a sixth wiring 17 anda start signal SP3 may be input to the pulse output circuit in the N-thstage through a eighth wiring 18. Alternatively, signals internallygenerated may be input to the pulse output circuits in the (N−1)-thstage and the N-th stage. For example, a pulse output circuit 10_(N+1)in the (N+1)-th stage and a pulse output circuit 10_(N+2) in the(N+2)-th stage (the circuits also referred to as pulse output circuitsin dummy stages) which do not contribute to pulse output to the pixelportion may be provided, a start signal SP2 may be input to the pulseoutput circuit 10_(N+1) in the (N+1)-th stage and a start signal SP3 maybe input to the pulse output circuit 10_(N+2) in the (N+2)-th stage.

Each of the first clock signal (CK1) to the fourth clock signal (CK4) isa digital signal whose level is repeatedly switched between high leveland low level. The first to the fourth clock signals (CK1) to (CK4) aredelayed by ¼ period sequentially. In this embodiment, driving of thepulse output circuit or the like is controlled with the first to fourthclock signals (CK1) to (CK4).

Each of the first to N-th pulse output circuits 10_1 to 10_N has a firstinput terminal 21, a second input terminal 22, a third input terminal23, a fourth input terminal 24, a fifth input terminal 25, a firstoutput terminal 26, and a second output terminal 27 (see FIG. 5B).

The first input terminal 21, the second input terminal 22, and the thirdinput terminal 23 are electrically connected to three wirings among thefirst to fourth wirings 11 to 14. For example, in the first pulse outputcircuit 10_1 in FIGS. 5A and 5B, the first input terminal 21 iselectrically connected to the first wiring 11, the second input terminal22 is electrically connected to the second wiring 12, and the thirdinput terminal 23 is electrically connected to the third wiring 13. Inthe second pulse output circuit 102, the first input terminal 21 iselectrically connected to the second wiring 12, the second inputterminal 22 is electrically connected to the third wiring 13, and thethird input terminal 23 is electrically connected to the fourth wiring14.

In the first pulse output circuit 10_1 in FIGS. 5A and 5B, the startsignal is input through the fourth input terminal 24, the later-stagesignal (the second output signal of the third pulse output circuit 10_3)is input through the fifth input terminal 25, the first output signal isoutput through the first output terminal 26, and the second outputsignal is output through the second output terminal 27.

Next, an example of a specific circuit configuration of the pulse outputcircuit is described below with reference to FIG. 5C.

A pulse output circuit shown in FIG. 5C includes first to eleventhtransistors 31 to 41.

One of a source and a drain of the first transistor 31 is electricallyconnected to a power supply line 51, and a gate thereof is electricallyconnected to the fourth input terminal 24.

One of a source and a drain of the second transistor 32 is electricallyconnected to a power supply line 52.

One of a source and a drain of the third transistor 33 is electricallyconnected to the first input terminal 21, and the other of the sourceand the drain thereof is electrically connected to the first outputterminal 26.

One of a source and a drain of the fourth transistor 34 is electricallyconnected to the power supply line 52, the other of the source and thedrain thereof is electrically connected to the first output terminal 26,and a gate thereof is electrically connected to a gate of the secondtransistor 32.

One of a source and a drain of the fifth transistor 35 is electricallyconnected to the power supply line 52, the other of the source and thedrain thereof is electrically connected to the gate of the secondtransistor 32, and a gate thereof is electrically connected to thefourth input terminal 24.

One of a source and a drain of the sixth transistor 36 is electricallyconnected to the power supply line 51, the other of the source and thedrain thereof is electrically connected to the gate of the secondtransistor 32, and a gate thereof is electrically connected to the fifthinput terminal 25.

One of a source and a drain of the seventh transistor 37 is electricallyconnected to the power supply line 51, and a gate thereof iselectrically connected to the third input terminal 23.

One of a source and a drain of the eighth transistor 38 is electricallyconnected to the gate of the second transistor 32, the other of thesource and the drain thereof is electrically connected to the other ofthe source and the drain of the seventh transistor 37, and a gatethereof is electrically connected to the second input terminal 22.

One of a source and a drain of the ninth transistor 39 is electricallyconnected to the other of the source and the drain of the firsttransistor 31 and the other of the source and the drain of the secondtransistor 32, the other of the source and the drain thereof iselectrically connected to a gate of the third transistor 33, and a gatethereof is electrically connected to the power supply line 51.

One of a source and a drain of the tenth transistor 40 is electricallyconnected to the first input terminal 21, the other of the source andthe drain thereof is electrically connected to the second outputterminal 27, and a gate thereof is electrically connected to the otherof the source and the drain of the ninth transistor 39.

One of a source and a drain of the eleventh transistor 41 iselectrically connected to the power supply line 52, the other of thesource and the drain thereof is electrically connected to the secondoutput terminal 27, and a gate thereof is electrically connected to thegate of the second transistor 32.

In FIG. 15C, a portion where the gate of the third transistor 33, thegate of the tenth transistor 40, and the other of the source and thedrain of the ninth transistor 39 are connected to one another isreferred to as a node NA. In addition, a portion where the gate of thesecond transistor 32, the gate of the fourth transistor 34, the other ofthe source and the drain of the fifth transistor 35, the other of thesource and the drain of the sixth transistor 36, the other of the sourceand the drain of the eighth transistor 38, and the gate of the eleventhtransistor 41 are connected to one another is referred to as a node NB.

For example, in the first pulse output circuit 10_1, the first clocksignal CK1 is input through the first input terminal 21, the secondclock signal CK2 is input through the second input terminal 22, thethird clock signal CK3 is input through the third input terminal 23, thestart signal SP1 is input to the fourth input terminal 24, and a signaloutput through the first output terminal 26 of the third pulse outputcircuit 10_3 is input through the fifth input terminal 25. In addition,the first pulse output circuit 10_1 outputs a pulse signal through thefirst output terminal 26, and outputs a signal OUT(1) through the secondoutput terminal 27.

A timing chart of the signals in the shift register shown in FIGS. 5A to5C is illustrated in FIG. 6. In the case where the shift register isincluded in a scan line driver circuit, a period 61 in FIG. 6corresponds to a vertical retrace period and a period 62 corresponds toa gate selection period.

The liquid crystal display device to which the driving method of theliquid crystal display device which is one embodiment of the presentinvention can be applied can display a still image and a moving imageand display a still image by refresh operation without constantlyoperating the driver circuit portion. A signal or voltage supplyoperation to each wiring in the case where a moving image is displayedafter the still image display is displayed and a stop operation of asignal or voltage supply to each wiring of the driver circuit portion atthe time of a rewriting operation (refresh operation) of a voltageapplied to the liquid crystal element in the scan line driver circuit orthe signal line driver circuit using the shift register shown as anexample in FIGS. 5A to 5C is described below with reference to FIG. 7.FIG. 7 illustrates changes in voltages, before and after a first frameperiod (T1), of a wiring for supplying the high power supply voltage(VDD), a wiring for supplying the low power supply voltage (VSS), awiring for supplying the start signal (SP1), and wirings for supplyingthe first to fourth clock signals CK1 to CK4 to the shift register.

As shown in FIG. 7, according to the operation of the shift register ofthis embodiment, a period in which the high power supply voltage andcontrol signals such as the first to fourth clock signals and the startsignal are supplied and a period in which the control signals are notsupplied exist. The first frame period T1 in FIG. 7 corresponds to theperiod in which the control signals are supplied, in other words, aperiod in which a moving image is displayed or a period in which refreshoperation is performed. A second frame period T2 in FIG. 7 correspondsto the period in which the control signals are not supplied, in otherwords, a period in which a still image is displayed.

In FIG. 7, a period in which the high power supply voltage is suppliedis not only in the first frame period but also in a part of the secondframe period which moves to/from the first frame. Also in FIG. 7, aperiod in which one or more of the first to fourth clock signals issupplied is after the supply of the high power supply voltage is startedand before the supply is stopped.

Further, as shown in FIG. 7, the wirings for supplying the first tofourth clock signals CK1 to CK4 may be set such that they become highonce before the first frame period, then start oscillating theirrespective clock signals CK1 to CK4 at a constant frequency, and theybecome low once after the first frame period, and then stop oscillatingthe clock signals.

As described above, in the shift register of this embodiment, the supplyof the high power supply voltage and the control signals such as thefirst to fourth clock signals and the start signal to the shift registeris stopped in the beginning or the ending of the second frame period. Inaddition, in the period in which the supply of the high power supplyvoltage and the control signals such as the first to fourth clocksignals and the start signal is stopped, an output of the pulse signalfrom the shift register is also stopped. Accordingly, power consumptionof the shift register and power consumption of the pixel portion whichis driven by the shift register can be reduced.

In the period in which the supply of the high power supply voltage tothe shift register is stopped, the voltage of the wiring for supplyingthe high power supply voltage can have the same value as the low powersupply voltage (Vss) as shown in FIG. 7. The wiring for supplying thehigh power supply voltage may be in a floating state, thereby stoppingthe supply of the high power supply voltage.

When the voltage of the wiring for supplying the high power supplyvoltage is increased, that is, the voltage is increased from the lowpower supply voltage to the high power supply voltage before the firstframe period, it is preferable to control the voltage of the wiring soas to change gradually. This is because, in the case where the voltageof the wiring for supplying the high power supply voltage is increased,a steep change of the voltage of the wiring may become noise, which mayfluctuate the waveform of the pulse signal output from the shiftregister and may change the voltage applied to the liquid crystalelement due to this fluctuation of the waveform, which leads to a changeof a still image. In view of the above, FIG. 7 illustrates an example inwhich a rise in the voltage of the wiring for supplying the high powersupply voltage is more gradual than a fall in the same. Particularly, inthe shift register of this embodiment, the supply of the high powersupply voltage is stopped or restarted as appropriate in a period inwhich a still image is displayed on the pixel portion. In other words,since a fluctuation in the voltage for supplying the high power supplyvoltage which enters the pixel portion as a noise leads to deteriorationof the display quality, it is important to perform control such that thefluctuation in the voltage for supplying the high power supply voltage(particularly, an increase in the voltage) does not enter the pixelportion as a noise.

This embodiment can be combined with or replaced by any of otherembodiments as appropriate.

(Embodiment 3)

In Embodiment 3, an example of a transistor which is applicable as thetransistor included in the liquid crystal display device described inEmbodiment 1 will be described.

A transistor in this embodiment is described below with reference toFIGS. 8A to 8D. FIGS. 8A to 8D are views for illustrating the transistordescribed in Embodiment 1.

A process for manufacturing a transistor 410 over a substrate 400 isdescribed below with reference to FIGS. 8A to 8D.

First, a conductive film is formed over the substrate 400 having aninsulating surface, a resist mask is formed over the conductive film bya first photolithography process, and then, the conductive film isetched using the resist mask, so that a gate electrode layer 411 isformed. After that, the resist mask is removed. The resist mask may beformed by an inkjet method. Formation of a resist mask by an inkjetmethod does not use a photomask, so that the manufacturing cost can bereduced.

Although there is no particular limitation on a substrate which can beused as the substrate 400 having an insulating surface, it is necessarythat the substrate have at least enough heat resistance to a heattreatment performed later. For example, as the substrate 400, a glasssubstrate of barium borosilicate glass, aluminoborosilicate glass, orthe like can be used. As a glass substrate, if the temperature of theheat treatment performed later is high, a glass substrate whose strainpoint is 730° C. or higher is preferably used.

In the transistor in this embodiment, an insulating film which functionsas a base film may be provided between the substrate 400 and the gateelectrode layer 411. The base film has a function of preventingdiffusion of an impurity element from the substrate 400, and can beformed with a single film or a plurality of films using one or more of asilicon nitride film, a silicon oxide film, a silicon nitride oxidefilm, and a silicon oxynitride film.

The gate electrode 411 can be formed with a single layer or a pluralityof stacked layers using one or more selected from metals such asmolybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper,neodymium, scandium, and the like and alloy materials using such a metalas a main component.

For example, the gate electrode 411 can be formed with a stacked layerin which a molybdenum layer is stacked on an aluminum layer, a stackedlayer in which a molybdenum layer is stacked on a copper layer, astacked layer in which a titanium nitride layer or a tantalum nitridelayer is stacked on a copper layer, or a stacked layer in which atitanium nitride layer and a molybdenum layer are stacked.Alternatively, the gate electrode 411 can be formed with a stacked layerin which a tungsten layer or a tungsten nitride layer, an alloy layer ofaluminum and silicon or an alloy layer of aluminum and titanium, and atitanium nitride layer or a titanium layer are stacked.

Next, a gate insulating layer 402 is formed over the gate electrodelayer 411.

The gate insulating layer 402 can be formed to have a single-layerstructure or a stacked-layer structure including one or more selectedfrom a silicon oxide layer, a silicon nitride layer, a siliconoxynitride layer, a silicon nitride oxide layer, and an aluminum oxidelayer by a plasma CVD method, a sputtering method, or the like. Forexample, a silicon oxynitride layer may be formed using a deposition gascontaining silane (SiH₄), oxygen, and nitrogen by a plasma CVD method. Alayer including a high-k material such as hafnium oxide (HfO_(x)) ortantalum oxide (TaO_(x)) can be used as the gate insulating layer 402.The thickness of the gate insulating layer 402 is greater than or equalto 100 nm and less than or equal to 500 nm in this embodiment; in thecase of a stacked-layer structure, for example, the gate insulatinglayer 402 is formed by stacking a first gate insulating layer with athickness greater than or equal to 50 nm and less than or equal to 200nm and a second gate insulating layer with a thickness greater than orequal to 5 nm and less than or equal to 300 nm.

In this embodiment, a silicon oxynitride layer having a thickness of 100nm is formed by a plasma CVD method as the gate insulating layer 402.

Note that as the gate insulating layer 402, a silicon oxynitride filmmay be formed using a high-density plasma apparatus. Here, thehigh-density plasma apparatus refers to an apparatus which can realize aplasma density higher than or equal to 1×10¹¹/cm³. For example, plasmais generated by applying a microwave power higher than or equal to 3 kWand lower than or equal to 6 kW so that an insulating film is formed.

For example, silane (SiH₄), nitrous oxide (N₂O), and a rare gas areintroduced into a chamber as a source gas and a pressure higher than orequal to 10 Pa and lower than or equal to 30 Pa is applied, which isfollowed by application of high-density plasma, so that an insulatingfilm is formed over the substrate having an insulating surface, such asa glass substrate. After that, the supply of silane (SiH₄) may bestopped, and nitrous oxide (N₂O) and a rare gas may be introducedwithout exposure of the insulating film to the air to perform plasmatreatment on a surface thereof. The insulating film formed through theabove process can ensure the reliability even if the thickness is assmall as 100 nm.

At the time of the formation of the gate insulating layer 402, the flowratio of silane (SiH₄) to nitrous oxide (N₂O) which are introduced intothe chamber is in the range of 1:10 to 1:200. As the rare gas which isintroduced into the chamber, helium, argon, krypton, xenon, or the likecan be used; of these, argon, which is inexpensive, is preferably used.

Since the insulating film formed using the high-density plasma apparatuscan have uniform thickness, the insulating film has high step coverage.Further, as for the insulating film formed using the high-density plasmaapparatus, the thickness of a thin film can be controlled precisely.

The insulating film formed through the above process is greatlydifferent from the insulating film formed using a conventional parallelplate plasma CVD apparatus. The etching rate of the insulating filmformed through the above process is lower than that of the insulatingfilm formed using the conventional parallel plate plasma CVD apparatusby 10% or more or 20% or more with the use of the same etchant, whichmeans that the insulating film formed using the high-density plasmaapparatus is a dense film.

An oxide semiconductor (a highly purified oxide semiconductor) layerwhich is made to be intrinsic (i-type) or substantially intrinsic in alater step is highly sensitive to an interface state and interfacecharge; thus, an interface between the oxide semiconductor layer and thegate insulating layer is important. For that reason, the gate insulatinglayer that is to be in contact with the highly-purified oxidesemiconductor needs to have high quality. For example, an insulatingfilm is formed using a high-density plasma CVD apparatus with microwaves(2.45 GHz), so that a high-quality insulating film which is dense andhas high dielectric strength voltage can be formed. The highly purifiedoxide semiconductor layer is in contact with the high-quality gateinsulating layer, whereby the interface states can be reduced andinterface properties can be favorable. As described above, it isimportant to form a favorable interface with lower interface statedensity between the oxide semiconductor layer and the gate insulatinglayer in addition to forming a gate insulating layer with high filmquality.

Next, an oxide semiconductor film 430 is formed to a thickness greaterthan or equal to 2 nm and less than or equal to 200 nm over the gateinsulating layer 402. Before the oxide semiconductor film 430 is formedby a sputtering method, powdery substances (also referred to asparticles or dust) which are attached on a surface of the gateinsulating layer 402 are preferably removed by reverse sputtering inwhich an argon gas is introduced and plasma is generated. The reversesputtering refers to a method in which, without application of a voltageto a target side, an RF power source is used for application of avoltage to a substrate side in an argon atmosphere so that plasma isgenerated to modify the surface of the substrate. Instead of the argonatmosphere, a nitrogen atmosphere, a helium atmosphere, an oxygenatmosphere, or the like may be used.

As the oxide semiconductor film 430, any of the following oxidesemiconductor films can be used: an In—Ga—Zn—O-based oxide semiconductorfilm; an In—Sn—O-based oxide semiconductor film; an In—Sn—Zn—O-basedoxide semiconductor film; an In—Al—Zn—O-based oxide semiconductor film;a Sn—Ga—Zn—O-based oxide semiconductor film; an Al—Ga—Zn—O-based oxidesemiconductor film; an Sn—Al—Zn—O-based oxide semiconductor film; anIn—Zn—O-based oxide semiconductor film; a Sn—Zn—O-based oxidesemiconductor film; an Al—Zn—O-based oxide semiconductor film; anIn—O-based oxide semiconductor film; a Sn—O-based oxide semiconductorfilm; and a Zn—O-based oxide semiconductor film. For example, in thecase of using an In—Ga—Zn—O-based oxide semiconductor film, it ispreferable that the thickness thereof is greater than or equal to 5 nmand less than or equal to 200 nm. In this embodiment, a 20-nm-thickIn—Ga—Zn—O-based oxide semiconductor film is formed by a sputteringmethod using an In—Ga—Zn—O-based metal oxide target as the oxidesemiconductor film 430. A cross-sectional view in this step is FIG. 8A.The oxide semiconductor film 430 can be formed by a sputtering method ina rare gas (typically argon) atmosphere, an oxygen atmosphere, or anatmosphere including a rare gas (typically argon) and oxygen. In thecase of using a sputtering method, it is preferable to use a targetcontaining SiO₂ at 2 wt % to 10 wt % to make SiO_(x) (x>0) that inhibitscrystallization be contained in the oxide semiconductor film in order tosuppress crystallization at the time of heat treatment for dehydrationor dehydrogenation in a later step.

In this embodiment, the oxide semiconductor film 430 is formed asfollows: a metal oxide target containing In, Ga, and Zn(In₂O₃:Ga₂O₃:ZnO=1:1:1 [in a molar ratio], In:Ga:Zn=1:1:0.5 [in anatomic ratio]) is used; the distance between the substrate and thetarget is 100 mm; the pressure is 0.2 Pa; the direct current (DC) poweris 0.5 kW; and the atmosphere is a mixed atmosphere of argon and oxygen(argon:oxygen=30 sccm:20 sccm, the oxygen flow rate is 40%). A pulsedirect current (DC) power supply is preferably used because powdersubstances generated at the time of film deposition can be reduced andthe film thickness can be made uniform. As the metal oxide targetcontaining In, Ga, and Zn, a target having a composition ratio ofIn:Ga:Zn=1:1:1 [in an atomic ratio] or a target having a compositionratio of In:Ga:Zn=1:1:2 [in an atomic ratio] can alternatively be used.

Examples of the sputtering method include an RF sputtering method usinga high-frequency power source as a sputtering power source, a DCsputtering method using a DC power source as a sputtering power source,and a pulsed DC sputtering method in which a bias is applied in a pulsedmanner. The RF sputtering method is mainly used in the case of formingan insulating film, and the DC sputtering method is mainly used in thecase of forming a metal film.

Further, there is a sputtering method using a multi-source sputteringapparatus in which a plurality of targets of different materials can beset. With the multi-source sputtering apparatus, films of differentmaterials can be deposited to be stacked in the same chamber, and pluralkinds of materials can be deposited by electric discharge at the sametime in the same chamber.

In addition, there are a magnetron sputtering method using a sputteringapparatus provided with a magnet system inside the chamber, and an ECRsputtering method using plasma generated with the use of microwaveswithout using glow discharge.

Further, as other examples of the sputtering method, there are also areactive sputtering method in which a target substance and a sputteringgas component are chemically reacted with each other during filmformation to deposit a thin compound film thereof, and a bias sputteringmethod in which a voltage is also applied to a substrate during filmformation.

Next, a resist mask is formed over the oxide semiconductor film 430 by asecond photolithography process, and then, the oxide semiconductor film430 is etched using the resist mask, so that the oxide semiconductorfilm 430 is processed into an island-shaped oxide semiconductor layer.After that, the resist mask is removed.

Next, dehydration or dehydrogenation of the oxide semiconductor layer isperformed. The dehydration or dehydrogenation is carried out byconducting a first heat treatment and a second heat treatment. Thetemperature of the first heat treatment is higher than or equal to 400°C. and lower than or equal to 750° C., preferably higher than or equalto 400° C. and lower than the strain point of the substrate. In thisembodiment, the substrate is introduced into an electric furnace whichis one of heat treatment apparatuses, and heat treatment is performed onthe oxide semiconductor layer in a nitrogen atmosphere at 450° C. forone hour. Then, the oxide semiconductor layer is cooled without exposureto the air in order to prevent entry of water and hydrogen into theoxide semiconductor layer, whereby an oxide semiconductor layer 431 isobtained (FIG. 8B).

The heat treatment apparatus is not limited to an electric furnace, andmay be provided with a device for heating an object by heat conductionor thermal radiation from a heater such as a resistance heater. Forexample, an RTA (rapid thermal anneal) apparatus such as a GRTA (gasrapid thermal anneal) apparatus or an LRTA (lamp rapid thermal anneal)apparatus can be used. The LRTA apparatus is an apparatus for heating anobject by radiation of light (an electromagnetic wave) emitted from alamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, acarbon arc lamp, a high pressure sodium lamp, or a high pressure mercurylamp. The GRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas. As the gas, an inert gas which does not react withthe object by heat treatment, like nitrogen or a rare gas such as argonis used.

For example, as the first heat treatment, GRTA may be performed in whichthe substrate is transferred into a chamber filled with an inert gasheated to a high temperature as high as 650° C. to 700° C., heated forseveral minutes, and taken out from the inert gas. With GRTA,high-temperature heat treatment for a short period of time can beachieved.

It is preferable that in the first heat treatment, water, hydrogen, orthe like be not contained in nitrogen or the rare gas such as helium,neon, or argon. Alternatively, the purity of nitrogen or the rare gassuch as helium, neon, or argon introduced in the heat treatmentapparatus is preferably 6N (99.9999%) or higher, far preferably 7N(99.99999%) or higher (that is, the concentration of impurities ispreferably 1 ppm or less, far preferably 0.1 ppm or less).

The first heat treatment may be performed on the oxide semiconductorfilm 430 before being processed into the island-shaped oxidesemiconductor layer. In that case, after the first heat treatment, thesubstrate is extracted from the heat treatment apparatus, and then thesecond photolithography process is performed.

The first heat treatment for dehydration or dehydrogenation of the oxidesemiconductor layer may be performed at any of the following timings:after the oxide semiconductor layer is formed; and after a sourceelectrode layer and a drain electrode layer are formed over the oxidesemiconductor layer.

In the case where an opening is formed in the gate insulating layer 402,the opening may be formed in the gate insulating layer 402 before orafter the dehydration or dehydrogenation of the oxide semiconductor film430

The etching of the oxide semiconductor film 430 in this embodiment isnot limited to wet etching; dry etching may be employed.

As an etching gas for the dry etching, a gas containing chlorine(chlorine-based gas such as chlorine (Cl₂), boron trichloride (BCl₃),silicon tetrachloride (SiCl₄), or carbon tetrachloride (CCl₄)) ispreferably used.

Alternatively, a gas containing fluorine (fluorine-based gas such ascarbon tetrafluoride (CF₄), sulfur hexafluoride (SF₆), nitrogentrifluoride (NF₃), or trifluoromethane (CHF₃)); hydrogen bromide (HBr);oxygen (O₂); any of these gases to which a rare gas such as helium (He)or argon (Ar) is added; or the like can be used as the etching gas forthe dry etching.

As the dry etching method, a parallel plate RIE (reactive ion etching)method or an ICP (inductively coupled plasma) etching method can beused. In order to etch into an appropriate shape, etching conditions(electric power applied to an electrode coil, electric power applied toan electrode on a substrate side, the temperature of the electrode onthe substrate side, or the like) are adjusted as appropriate.

As an etchant used for the wet etching, a mixed solution of phosphoricacid, acetic acid, and nitric acid, or the like can be used. ITO07N(produced by KANTO CHEMICAL CO., INC.) may be used as well.

The etchant after the wet etching is removed together with the etchedmaterial by cleaning. The waste liquid including the etchant and theetched material may be purified and any material contained in the etchedmaterial may be reused. For example, indium contained in the oxidesemiconductor layer is collected from the waste liquid after the etchingand recycled, so that resources can be effectively used and cost can bereduced.

Also in the wet etching, in order to etch into an appropriate shape,etching conditions (an etchant, etching time, temperature, and the like)are adjusted as appropriate depending on the material.

Next, a metal conductive film is formed over the gate insulating layer402 and the oxide semiconductor layer 431. For example, a metalconductive film may be formed by a sputtering method or a vapordeposition method. As a material of the metal conductive film, it ispossible to use an element selected from aluminum (Al), chromium (Cr),copper (Cu), tantalum (Ta), titanium (Ti), molybdenum (Mo), and tungsten(W), an alloy containing any of the elements as a component, an alloycombining the elements, or the like. Alternatively, one or morematerials selected from manganese (Mn), magnesium (Mg), zirconium (Zr),beryllium (Be), and yttrium (Y) may be used. Further, the metalconductive film may have a single-layer structure or a stacked-layerstructure of two or more layers. For example, as examples of the metalconductive film, the following can be given: a single layer of analuminum film containing silicon; a single layer of a copper film or afilm containing copper as a main component; a stacked layer in which atitanium film is stacked on an aluminum film; a stacked layer in which acopper film is stacked on a tantalum nitride film or a copper nitridefilm; a stacked layer in which an aluminum film is stacked on a titaniumfilm, and a titanium film is stacked on the aluminum film; and the like.Alternatively, a film, an alloy film, or a nitride film which containsaluminum (Al) and one or a plurality of elements selected from titanium(Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr),neodymium (Nd), and scandium (Sc) may be used.

In the case where the first heat treatment is performed after formationof the metal conductive film, it is preferable that the metal conductivefilm have heat resistance enough to withstand the first heat treatment.

Next, a resist mask is formed over the metal conducive film by a thirdphotolithography process, and then, the metal conducive film is etchedusing the resist mask, so that a source electrode layer 415 a and adrain electrode layer 415 b are formed. After that, the resist mask isremoved (see FIG. 8C).

Materials and etching conditions are adjusted as appropriate so that theoxide semiconductor layer 431 is not removed by the etching of the metalconductive film.

In this embodiment, a titanium film is used as the metal conductivefilm, an In—Ga—Zn—O-based oxide semiconductor layer is used as the oxidesemiconductor layer 431, and an ammonia hydrogen peroxide solution (amixture of ammonia, water, and a hydrogen peroxide solution) is used asan etchant of the titanium film.

The third photolithography process may also etch part of the oxidesemiconductor layer 431 so that a groove (a depressed portion) is formedin the oxide semiconductor layer. The resist mask used in this step maybe formed by an inkjet method. Formation of the resist mask by an inkjetmethod needs no photomask; thus, manufacturing cost can be reduced.

In order to reduce the number of photomasks and the number of steps inthe photolithography process, the etching may be performed with the useof a resist mask formed using a multi-tone mask which is a photomaskthrough which light is transmitted to have a plurality of intensities.Since the resist mask formed using a multi-tone mask has a plurality ofthicknesses and can be further changed in shape by ashing, the resistmask can be used in a plurality of etching steps to form differentpatterns. Therefore, a resist mask corresponding to at least two kindsof different patterns can be formed by one multi-tone mask. Thus, thenumber of photomasks can be reduced, which leads to simplification ofthe manufacturing process.

Next, plasma treatment using a gas of nitrous oxide (N₂O), nitrogen(N₂), or argon (Ar) is performed. By this plasma treatment, adsorbedwater and the like attached to an exposed surface of the oxidesemiconductor layer are removed. Plasma treatment may be performed usinga mixture gas of oxygen and argon as well.

An oxide insulating layer 416 which serves as a protective insulatingfilm and is in contact with part of the oxide semiconductor layer isnext formed without exposure to the air after the plasma treatment.

The oxide insulating layer 416 can be formed to a thickness of at least1 nm by a method with which impurities such as water or hydrogen do notenter the oxide insulating layer 416, such as a sputtering method, asappropriate. Hydrogen contained in the oxide insulating layer 416 entersthe oxide semiconductor layer, which makes the resistance of thebackchannel of the oxide semiconductor layer 431 low (makes thebackchannel have an n-type conductivity) to form a parasitic channel.Therefore, it is important that a formation method in which hydrogen isnot used is employed in order to form the oxide insulating layer 416containing as little hydrogen as possible.

In this embodiment, a 200-nm-thick silicon oxide film is formed as theoxide insulating layer 416 by a sputtering method. The substratetemperature in the film formation may be higher than or equal to roomtemperature and lower than or equal to 300° C. and is 100° C. in thisembodiment. The formation of a silicon oxide film by a sputtering methodcan be performed in a rare gas (typically, argon) atmosphere, an oxygenatmosphere, or an atmosphere of a rare gas (typically, argon) andoxygen. A silicon oxide target or a silicon target can be used as atarget. For example, the silicon oxide film can be formed using asilicon target by a sputtering method in an atmosphere containing oxygenand nitrogen.

Next, a second heat treatment (preferably at a temperature higher thanor equal to 200° C. and lower than or equal to 400° C., e.g., at atemperature higher than or equal to 250° C. and lower than or equal to350° C.) is performed in an inert gas atmosphere or an oxygen gasatmosphere. For example, the second heat treatment is performed in anitrogen atmosphere at 250° C. for one hour. Through the second heattreatment, part of the oxide semiconductor layer (a channel formationregion) is heated while being in contact with the oxide insulating layer416. Accordingly, oxygen is supplied to the part of the oxidesemiconductor layer 431 (the channel formation region).

As described above, the oxide semiconductor layer is subjected to thesecond heat treatment for dehydration or dehydrogenation by which thepart of the oxide semiconductor layer (channel formation region) isselectively made in an oxygen-excess state. Consequently, a channelformation region 413 which overlaps with the gate electrode layer 411becomes an i-type, and a low-resistance region 414 a whose electricalresistance is lower than that of the channel formation region 413 andwhich overlaps with the source electrode layer 415 a and alow-resistance region 414 b whose electrical resistance is lower thanthat of the channel formation region 413 and which overlaps with thedrain electrode layer 415 b are formed in a self-aligned manner. Throughthe above-described process, a transistor 410 is formed.

For example, with a gate bias-temperature stress test (BT test) at 85°C. with 2×10⁶ V/cm for 12 hours, an impurity contained in an oxidesemiconductor layer causes the following phenomenon: a bond between theimpurity and a main component of the oxide semiconductor layer iscleaved by a high electrical field (B: bias) and high temperature (T:temperature), and a generated dangling bond causes drift of thethreshold voltage (Vth). On the other hand, in the case where impuritiesin an oxide semiconductor layer, particularly hydrogen or water, areremoved as much as possible and a high-quality gate insulating layerwhich is dense and has high dielectric strength voltage is formed usingthe above-described high-density plasma CVD apparatus to provide highinterface properties between the gate insulating layer and the oxidesemiconductor layer, a transistor which is stable even under the BT testcan be provided.

Heat treatment may be further performed, after the second heattreatment, at a temperature higher than or equal to 100° C. and lowerthan or equal to 200° C. for a period longer than or equal to 1 hour andshorter than or equal to 30 hours in the air. In this embodiment, theheat treatment is performed at 150° C. for 10 hours. This heat treatmentmay be performed at a fixed temperature. Alternatively, the followingchange in the heating temperature may be repeated plural times: theheating temperature is increased from room temperature to a temperaturehigher than or equal to 100° C. and lower than or equal to 200° C. andthen decreased to room temperature. This heat treatment may be performedunder a reduced pressure. Under the reduced pressure, the heat treatmenttime can be shortened.

The formation of the low-resistance region 414 b in the oxidesemiconductor layer so as to overlap with the drain electrode layer 415b can improve the reliability of the transistor. Specifically, by theformation of the low-resistance region 414 b, a structure can beobtained in which the conductivity of the transistor can be variedgradually from the drain electrode layer 415 b to the channel formationregion 413 through the low-resistance drain region 414 b.

In the case where the thickness of the oxide semiconductor layer is assmall as less than or equal to 15 nm, the low-resistance region in theoxide semiconductor layer is formed wholly in the thickness direction;in the case where the thickness of the oxide semiconductor layer is aslarge as greater than or equal to 30 nm and less than or equal to 50 nm,part of the oxide semiconductor layer, which is in contact with thesource or drain electrode layer and in a peripheral thereof is made tobe the low-resistance region in which the resistance is lower, and partof the oxide semiconductor layer, which is close to the gate insulatinglayer can be made to be an i-type.

A protective insulating layer may be formed over the oxide insulatinglayer 416. For example, a silicon nitride film is formed by an RFsputtering method. The RF sputtering method, which achieves high massproductivity, is preferable as a formation method of the protectiveinsulating layer. The protective insulating layer is a layer in whichimpurities such as moisture, a hydrogen ion, or an OH⁻ ion are notcontained or contained as little as possible. An inorganic insulatingfilm which blocks entry of them can be formed as the protectiveinsulating layer. As the inorganic insulating film, a silicon nitridefilm, an aluminum nitride film, a silicon nitride oxide film, analuminum oxynitride film, or the like can be used. In this embodiment, asilicon nitride film is formed as a protective insulating layer 403 (seeFIG. 8D).

In this manner, the transistor included in the liquid crystal displaydevice of the above embodiment can be manufactured. One embodiment ofthe present invention is not limited to the transistor; a multi-gatetransistor having a plurality of channel formation regions can be usedas the transistor included in the liquid crystal display device of theabove embodiment, as appropriate. A top-gate transistor can be used asthe transistor included in the liquid crystal display device of theabove embodiment, as well. A channel-etched transistor, a channel-stoptransistor, a bottom-contact transistor, or the like can also be used asthe transistor included in the liquid crystal display device of theabove embodiment.

This embodiment can be combined with or replaced by any of otherembodiments as appropriate.

(Embodiment 4)

In Embodiment 4, an external appearance and a cross section of anexample of the liquid crystal display device described in the aboveembodiment will be described using FIGS. 9A to 9C. FIGS. 9A to 9Cillustrate examples of the liquid crystal display device of thisembodiment: FIGS. 9A and 9C are plan views and FIG. 9B is across-sectional view along line M-N in FIG. 9A or 9C.

In the liquid crystal display device shown in FIGS. 9A to 9C, a sealant4005 is provided so as to surround a pixel portion 4002 and a scan linedriver circuit 4004 which are provided over a first substrate 4001. Inaddition, a second substrate 4006 is provided over the pixel portion4002 and the scan line driver circuit 4004. Thus, the pixel portion 4002and the scan line driver circuit 4004 in addition to a liquid crystallayer 4008 are sealed between the first substrate 4001 and the secondsubstrate 4006 with the sealant 4005. Further, in the liquid crystaldisplay device shown in FIGS. 9A to 9C, a signal line driver circuit4003 which is formed using a single crystal semiconductor film or apolycrystalline semiconductor film over another substrate is mounted ina region that is different from the region surrounded by the sealant4005 over the first substrate 4001.

There is no particular limitation on the connection method of a drivercircuit which is separately formed; a COG method, a wire bonding method,a TAB method, or the like can be used. FIG. 9A illustrates an example inwhich the signal line driver circuit 4003 is mounted by a COG method,and FIG. 9C illustrates an example in which the signal line drivercircuit 4003 is mounted by a TAB method.

Each of the pixel portion 4002 and the scan line driver circuit 4004which are provided over the first substrate 4001 includes a plurality oftransistors. In FIG. 9B, a transistor 4010 included in the pixel portion4002 and a transistor 4011 included in the scan line driver circuit 4004are illustrated as an example. Insulating layers 4041, 4042, and 4021are provided over the transistors 4010 and 4011.

As any of the transistors 4010 and 4011, a transistor including an oxidesemiconductor layer functioning as a channel formation layer can be usedlike the liquid crystal display device of the above embodiment; forexample, the transistor described in Embodiment 3 can be used.

The transistor 4010 includes a gate electrode layer 4051, a gateinsulating layer 4020 provided over the gate electrode layer 4051, anoxide semiconductor layer 4052 provided over the gate electrode layer4051 with the gate insulating layer 4020 provided therebetween, and asource electrode layer 4053 and a drain electrode layer 4054 providedover the oxide semiconductor layer 4052.

The transistor 4011 includes a gate electrode layer 4061, the gateinsulating layer 4020 provided over the gate electrode layer 4061, anoxide semiconductor layer 4062 provided over the gate electrode layer4061 with the gate insulating layer 4020 provided therebetween, and asource electrode layer 4063 and a drain electrode layer 4064 providedover the oxide semiconductor layer 4062.

A conductive layer 4040 is provided over the insulating layer 4021 so asto overlap with a channel formation region of the oxide semiconductorlayer 4062 in the transistor 4011. The provision of the conductive layer4040 so as to overlap with the channel formation region of the oxidesemiconductor layer 4062 can reduce the amount of shift in the thresholdvoltage of the transistor 4011 by external stress. The conductive layer4040 may have the same voltage as or have voltage different from that ofthe gate electrode layer 4061 of the transistor 4011 and can function asa second gat electrode layer. The voltage of the conductive layer 4040may be GND or 0V, or the conductive layer 4040 may be in a floatingstate. The conductive layer 4040 is not necessarily provided.

A pixel electrode layer 4030 is provided so as to be electricallyconnected to the source electrode layer 4053 or the drain electrodelayer 4054 of the transistor 4010 through an opening in the insulatinglayers 4041, 4042, and 4021. A counter electrode layer 4031 is providedto the second substrate 4006. A portion where the pixel electrode layer4030, the counter electrode layer 4031, and the liquid crystal layer4008 overlap with one another corresponds to a liquid crystal element4013. The pixel electrode layer 4030 and the counter electrode layer4031 are provided with an insulating layer 4032 and an insulating layer4033 serving as alignment films, respectively, and the liquid crystallayer 4008 is sandwiched between the pixel electrode layer 4030 and thecounter electrode layer 4031 with the insulating layers 4032 and 4033provided therebetween.

A light-transmitting substrate can be used as any of the first substrate4001 and the second substrate 4006; glass, ceramics, or plastics can beused. As the plastics, a fiberglass-reinforced plastics (FRP) plate, apoly(vinyl fluoride) (PVF) film, a polyester film, or an acrylic resinfilm can be used.

A spacer 4035 is provided between the insulating layers 4032 and 4033.The spacer 4035 is a columnar partition wall obtained by selectiveetching of an insulating film, and is provided in order to control thedistance (cell gap) between the pixel electrode layer 4030 and thecounter electrode layer 4031. A spherical spacer may be used as thespacer 4035.

The counter electrode layer 4031 is electrically connected to a commonvoltage line provided over the same substrate as the transistor 4010.With the use of a connection portion with the common voltage line (alsoreferred to as a common connection portion), the counter electrode layer4031 can be electrically connected to the common voltage line viaconductive particles arranged between the pair of substrates.

The sealant 4005 contains conductive particles.

In the liquid crystal display device of this embodiment, a liquidcrystal showing a blue phase for which an alignment film is not neededmay be used as a liquid crystal material of the liquid crystal layer4008. The blue phase is one of the liquid crystal phases, which appearsjust before a cholesteric phase changes into an isotropic phase whiletemperature of cholesteric liquid crystal is increased. Since the bluephase appears only within a narrow range of temperatures, a liquidcrystal composition containing a chiral agent at greater than or equalto 5 wt % is used for the liquid crystal layer 4008 in order to widenthe temperature range. The liquid crystal composition which includes aliquid crystal showing a blue phase and a chiral agent has a shortresponse time of 1 msec or less and has optical isotropy, which makesthe alignment process unnecessary, and the viewing-angle dependence issmall. In addition, since an alignment film does not need to be providedand rubbing treatment is also unnecessary, electrostatic dischargedamage caused by the rubbing treatment can be prevented and defects anddamage of the liquid crystal display device in the manufacturing processcan be reduced. Thus, productivity of the liquid crystal display devicecan be increased. A transistor including an oxide semiconductor layerparticularly has a possibility that electric characteristics of thetransistor may fluctuate significantly by the static electricity anddeviate from the design range. Therefore, it is more effective to use aliquid crystal material showing a blue phase for the liquid crystaldisplay device including a transistor including an oxide semiconductorlayer.

In the liquid crystal display device of this embodiment, a polarizingplate may be provided on the outer side of the substrate (on the viewerside) and a coloring layer and an electrode layer used in a displayelement may be sequentially provided on the inner side of the substrate;alternatively, the polarizing plate may be provided on the inner side ofthe substrate. The stacked-layer structure of the polarizing plate andthe coloring layer may be set as appropriate in accordance withmaterials of the polarizing plate and the coloring layer and thecondition of the manufacturing process. Further, a light-blocking layerserving as a black matrix may be provided in a portion other than thedisplay portion.

The insulating layer 4041 is in contact with parts of the oxidesemiconductor layers 4052 and 4062. A silicon oxide layer can be used asthe insulating layer 4041, for example.

The insulating layer 4042 is provided on and in contact with theinsulating layer 4041. A silicon nitride layer can be used as theinsulating layer 4042, for example.

The insulating layer 4021 is provided over the insulating layer 4042.The insulating layer 4021 functions as a planarization insulating layerfor reducing roughness of a surface of the transistor. An organicmaterial having heat resistance, such as a polyimide, an acrylic resin,a benzocyclobutene-based resin, a polyamide, or an epoxy resin can beused for the insulating layer 4021. Other than such organic materials,it is also possible to use a low-dielectric constant material (a low-kmaterial), a siloxane-based resin, PSG (phosphosilicate glass), BPSG(borophosphosilicate glass), or the like. The insulating layer 4021 maybe formed by stacking a plurality of insulating films formed of thesematerials.

There is no particular limitation on the formation method of theinsulating layer 4021. Depending on the material, the following methodcan be used: a sputtering method, an SOG method, a spin coating method,a dipping method, a spray coating method, or a droplet discharge method(e.g., an ink-jet method, a screen printing method, or an offsetprinting method) e.

The pixel electrode layer 4030 and the counter electrode layer 4031 canbe formed using a light-transmitting conductive material such as indiumtin oxide (ITO), indium zinc oxide (IZO) in which zinc oxide (ZnO) ismixed in indium oxide, a conductive material in which silicon oxide(SiO₂) is mixed in indium oxide, indium oxide, tin oxide, indium oxidecontaining tungsten oxide, indium zinc oxide containing tungsten oxide,indium oxide containing titanium oxide, indium tin oxide containingtitanium oxide, or the like. In the case where a light-transmittingproperty is not needed in a reflective liquid crystal display device,one or more kinds of materials selected from metals such as tungsten(W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium (V),niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel (Ni),titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), and silver(Ag); an alloy of these metals; and nitrides of these metals can beused.

A conductive composition containing a conductive high molecule (alsoreferred to as a conductive polymer) can be used for the pixel electrodelayer 4030 and the counter electrode layer 4031. The electrode layerformed using the conductive composition has preferably a sheetresistance of less than or equal to 10000 Ω/square and a transmittanceof 70% or more at a wavelength of 550 nm. Furthermore, the resistivityof the conductive high molecule contained in the conductive compositionis preferably 0.1 Ω·cm or less.

As the conductive high molecule, a so-called π-electron conjugatedconductive polymer can be used. Examples thereof are polyaniline or aderivative thereof, polypyrrole or a derivative thereof, polythiopheneor a derivative thereof, and a copolymer of two or more kinds ofmonomers of these materials.

Further, a variety of signals and voltages are supplied to the signalline driver circuit 4003 which is separately formed, the scan linedriver circuit 4004, or the pixel portion 4002 from an FPC 4018. The FPC4018 is electrically connected to a terminal electrode 4016 through aconnection terminal electrode 4015 and an anisotropic conductive film4019.

The connection terminal electrode 4015 is formed using the sameconductive film as the pixel electrode layer 4030 of the liquid crystalelement 4013, and the terminal electrode 4016 is formed using the sameconductive film as the source electrode layer 4053 or the drainelectrode layer 4054 of the transistor 4010.

Although FIGS. 9A to 9C illustrate an example in which the signal linedriver circuit 4003 is formed separately and mounted on the firstsubstrate 4001; however, one embodiment of the present invention is notlimited to this structure. The scan line driver circuit may beseparately formed and then mounted, or only part of the signal linedriver circuit or part of the scan line driver circuit may be separatelyformed and then mounted.

Further, a black matrix (a light-shielding layer); an optical member (anoptical substrate) such as a polarizing member, a retardation member, oran anti-reflection member; or the like can be provided as appropriatefor the liquid crystal display device shown in any of FIGS. 9A to 9C.For example, circular polarization may be obtained by using a polarizingsubstrate and a retardation substrate as the optical member. Inaddition, a backlight, a side light, or the like may be used as a lightsource.

In an active matrix liquid crystal display device, display patterns areformed on a screen by driving pixel electrode layers arranged in amatrix. Specifically, voltage is applied between a selected pixelelectrode layer and a counter electrode layer corresponding to the pixelelectrode layer, and thus, a liquid crystal layer disposed between thepixel electrode layer and the counter electrode layer is opticallymodulated. This optical modulation is recognized as a display pattern bya viewer.

Further alternatively, in order to improve the moving-imagecharacteristics of a liquid crystal display device, a driving techniquemay be employed in which a plurality of LED (light-emitting diode) lightsources or a plurality of EL light sources are used to form a surfacelight source as a backlight, and each light source of the surface lightsource is independently driven in a pulsed manner in one frame period.As the surface light source, three or more kinds of LEDs may be used andan LED emitting white light may be used. Since a plurality of LEDs canbe controlled independently, the light emission timing of LEDs can besynchronized with the timing at which a liquid crystal layer isoptically modulated. According to this driving technique, part of LEDscan be turned off, so that especially in the case of displaying an imagein which the proportion of a black image area in one screen is high,power consumption can be reduced.

By combining such a driving technique, the display characteristics ofthe liquid crystal display device described in the above embodiment canbe improved.

Since the transistor is easily broken due to static electricity or thelike, it is preferable that a protective circuit be provided over thesame substrate as the pixel portion and the driver circuit portion. Theprotective circuit is preferably formed using a non-linear elementincluding an oxide semiconductor layer. For example, protective circuitsare provided between the pixel portion and a scan line input terminaland between the pixel portion and a signal line input terminal. In thisembodiment, a plurality of protective circuits are provided so that thetransistor in the pixel and the like are not broken when a surge voltagedue to static electricity or the like is applied to a scan line, asignal line, or a capacitor bus line. Therefore, the protective circuitis formed so that charge is released to a common wiring when surgevoltage is applied to the protective circuit. Further, the protectivecircuit includes a non-linear element arranged in parallel to the scanline. The non-linear element includes a two-terminal element such as adiode or a three-terminal element such as a transistor. For example, thenon-linear element can be formed through the same process as thetransistor in the pixel portion. For example, characteristics similar tothose of a diode can be obtained by connecting a gate to a drain of thenon-linear element.

As a display mode of the liquid crystal display device of the presentinvention, the following can be used: a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, anaxially symmetric aligned micro-cell (ASM) mode, an opticallycompensated birefringence (OCB) mode, a ferroelectric liquid crystal(FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, or thelike.

There is no particular limitation on the liquid crystal of the liquidcrystal display device of this embodiment; a TN liquid crystal, an OCBliquid crystal, an STN liquid crystal, a VA liquid crystal, an ECBliquid crystal, a GH liquid crystal, a polymer dispersed liquid crystal,a discotic liquid crystal, or the like can be used. Among these, theliquid crystal display device of this embodiment is preferably anormally-black liquid crystal panel, for example, a transmissive liquidcrystal display device employing a vertical alignment (VA) mode. Someexamples are given as the vertical alignment mode; for example, an MVA(multi-domain vertical alignment) mode, a PVA (patterned verticalalignment) mode, an ASV mode, or the like can be employed.

In this manner, by using a transistor including an oxide semiconductorlayer functioning as a channel formation layer in the pixel portion ofthe liquid crystal display device of this embodiment, the display devicecan display a still image for a long period of time. Further, the drivercircuit portion can stop operating during a period in which a stillimage is displayed, whereby power consumption can be reduced.

This embodiment can be combined with or replaced by any of otherembodiments as appropriate.

(Embodiment 5)

In Embodiment 5, a liquid crystal display device added with a touchpanel function will be described as an example of the liquid crystaldisplay device described in the above embodiment.

FIGS. 10A and 10B illustrate examples of a structure of the liquidcrystal display device of this embodiment.

A liquid crystal display device shown in FIG. 10A includes a liquidcrystal display unit 6601 and a touch panel unit 6602 which is providedso as to overlap with the liquid crystal display unit 6601. The liquidcrystal display unit 6601 and the touch panel unit 6602 are attached toeach other with a housing (case) 6603.

The liquid crystal display device described in the above embodiment canbe applied as the liquid crystal display unit 6601.

As the touch panel unit 6602, a resistive touch panel, a surfacecapacitive touch panel, a projected capacitive touch panel, or the likecan be used as appropriate.

As shown in FIG. 10A, one example of the liquid crystal display deviceof this embodiment has a structure in which a liquid crystal displayunit and a touch panel unit which are separately manufactured overlapeach other. With this structure, manufacturing cost of the liquidcrystal display device added with a touch panel function can be reduced.

A liquid crystal display device 6604 shown in FIG. 10B includes aplurality of pixels 6605 in a display portion, and each pixel 6605includes a photosensor 6606 and a liquid crystal element 6607. Theliquid crystal display device 6604 shown in FIG. 10B reads data asfollows: an object data of which is to be read (e.g., a finger or a pen)is moved to touch or be close to the photosensor 6606 in the pixel 6605,and photocurrent is generated with the photosensor 6606 in accordancewith reflected light from the object. The liquid crystal display device6604 shown in FIG. 10B does not, unlike the liquid crystal displaydevice shown in FIG. 10A, involve an overlap of the touch panel unit6602, so that the thickness of the liquid crystal display device can bereduced. Further, in addition to the pixel portion 6605, a scan linedriver circuit 6608, a signal line driver circuit 6609, and aphotosensor driver circuit 6610 can be formed over the same substrate asthe pixel portion 6605, whereby the liquid crystal display device can bedownsized. The photosensor 6606 may be formed using amorphous silicon orthe like and overlapped with a transistor including an oxidesemiconductor.

By using a transistor including an oxide semiconductor layer functioningas a channel formation layer in the liquid crystal display device addedwith a touch panel function of this embodiment, the display device candisplay a still image for a long period of time. Further, the drivercircuit portion can stop operating during a period in which a stillimage is displayed, whereby power consumption can be reduced.

This embodiment can be combined with or replaced by any of otherembodiments as appropriate.

(Embodiment 6)

In Embodiment 6, an electronic book reader will be described as anexample of the liquid crystal display device described in the aboveembodiment.

An electronic book reader of this embodiment is described below usingFIG. 11. FIG. 11 illustrates an example of the electronic book reader ofthis embodiment.

The electronic book reader illustrated in FIG. 11 includes two housings,a housing 2701 and a housing 2703. The housings 2701 and 2703 areconnected by a hinge portion 2711 and can be opened or closed with thehinge portion 2711. With such a structure, the electronic book readercan operate like a paper book.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display different images. Oneimage may be displayed across both the display portions. In the case ofdisplaying different images, for example, text can be displayed on adisplay portion on the right side (the display portion 2705 in FIG. 11)and graphics can be displayed on a display portion on the left side (thedisplay portion 2707 in FIG. 11).

The example of the electronic book reader illustrated in FIG. 11 isprovided with an operation portion and the like for the housing 2701.For example, the housing 2701 is provided with a power switch 2721, anoperation key 2723, a speaker 2725, and the like. With the operation key2723, pages can be turned. Further, a keyboard, a pointing device, orthe like may also be provided on the same plane as the display portionof the housing. In addition, an external connection terminal (anearphone terminal, a USB terminal, a terminal connectable to a varietyof cables such as an AC adapter and a USB cable, or the like), arecording medium insertion portion, and the like may be provided on therear surface or the side surface of the housing. Further, a function ofan electronic dictionary may be provided for the electronic book readerillustrated in FIG. 11.

The electronic book reader of this embodiment may transmit and receivedata wirelessly. Through wireless communication, book data or the likecan be purchased and downloaded from an electronic book server.

The electronic book reader of this embodiment may have a power supplycircuit including a solar battery cell, a power storage device forcharging voltage output from the solar battery cell, and a DC converterfor converting a voltage charged in the power storage device intorespective voltages for circuits. Accordingly, an external power supplyis not needed, and thus the electronic book reader can be used for along period of time even at a place with no power supply, so thatconvenience can be improved. As the power storage device, for example,one or more of a lithium ion secondary battery, a lithium ion capacitor,an electric double-layer capacitor, a redox capacitor, and the like canbe used. For example, a lithium ion secondary battery and a lithium ioncapacitor are used together, whereby a power storage device which cancharge or discharge at high speed and can supply electric power for along time can be formed. There is no limitation the power storage deviceon the lithium ion secondary battery. For the power storage device, asecondary battery in which another alkali metal ion, alkaline earthmetal ion, or the like is used as a mobile ion may be used. There is nolimitation also on the lithium ion capacitor. For the power storagedevice, a capacitor in which another alkali metal ion, alkaline earthmetal ion, or the like is used as a mobile ion may be used.

By using a transistor including an oxide semiconductor layer functioningas a channel formation layer in the electronic book reader of thisembodiment, the display device can display a still image for a longperiod of time, which is particularly effective for displaying a stillimage for a long period of time on an electronic book reader. Further,the driver circuit portion can stop operating during a period in which astill image is displayed, whereby power consumption can be reduced.

This embodiment can be combined with or replaced by any of otherembodiments as appropriate.

(Embodiment 7)

In Embodiment 7, an electronic device which has the liquid crystaldisplay device described in the above embodiment in a display portionwill be described.

By applying the liquid crystal display device described in the aboveembodiment to display portions of a variety of electronic devices, avariety of functions in addition to a display function can be providedfor the electronic devices. Specific examples of the electronic devicein which the liquid crystal display device described in the aboveembodiment is applied to a display portion are described below usingFIGS. 12A to 12F. FIGS. 12A to 12F each illustrate an example of astructure of an electronic device of this embodiment.

FIG. 12A illustrates a personal digital assistant. The personal digitalassistant illustrated in FIG. 12A has at least a display portion 1001.The personal digital assistant illustrated in FIG. 12A can be combinedwith a touch panel or the like, and can be used as an alternative to avariety of his/her personal effects. For example, the display portion1001 is provided with an operation portion 1002, so that the personaldigital assistant can be used as a mobile phone. The operation portion1002 is not necessarily provided for the display portion 1001; anoperation buttons/operation bottoms may be provided anywhere for thepersonal digital assistant. Moreover, the personal digital assistant canbe used as a notepad or used as a handy scanner by using a documentinput-output function. Further, the liquid crystal display devicedescribed in the above embodiment can realize long intervals betweenwriting operations since a display period by one image-data writing islong. Therefore, by using the liquid crystal display device described inthe above embodiment for the personal digital assistant illustrated inFIG. 12A, eyestrain can be suppressed even when images can be seen onthe display portion for a long period of time, for example.

FIG. 12B illustrates an information guide terminal including anautomotive navigation system, for example. The information guideterminal illustrated in FIG. 12B has at least a display portion 1101,and can also have operation buttons 1102, an external input terminal1103, and the like. In-car temperatures change greatly in accordancewith the outside-air temperature, and sometimes exceed 50° C. The liquidcrystal display device described in the above embodiment, thecharacteristic change of which by the temperature is small, isparticularly effective under circumstances where the temperature greatlychanges such as the inside of a vehicle

FIG. 12C illustrates a laptop personal computer. The laptop personalcomputer illustrated in FIG. 12C has a housing 1201, a display portion1202, a speaker 1203, an LED lamp 1204, a pointing device 1205, aconnection terminal 1206, and a keyboard 1207. The liquid crystaldisplay device described in the above embodiment can realize longintervals between writing operations since a display period by oneimage-data writing is long. Therefore, by using the liquid crystaldisplay device described in the above embodiment for the laptop personalcomputer illustrated in FIG. 12C, eyestrain can be suppressed even whenimages can be seen on the display portion for a long period of time, forexample.

FIG. 12D illustrates a portable game machine. The portable game machineillustrated in FIG. 12D has a first display portion 1301, a seconddisplay portion 1302, a speaker 1303, a connection terminal 1304, an LEDlamp 1305, a microphone 1306, a recording medium reading portion 1307,operation buttons 1308, and a sensor 1309. Further, the liquid crystaldisplay device described in the above embodiment can realize longintervals between writing operations since a display period by oneimage-data writing is long. Therefore, by using the liquid crystaldisplay device described in the above embodiment for the portable gamemachine illustrated in FIG. 12D, eyestrain can be suppressed even whenimages can be seen on the display portion for a long period of time, forexample. Further, different images can be displayed on the first displayportion 1301 and the second display portion 1302; for example, a movingimage is displayed on one of them and a still image is displayed on theother. Accordingly, a signal or voltage supply to the driver circuitportion for the display portion where a still image is displayed can bestopped, whereby power consumption can be reduced.

FIG. 12E illustrates a stationary information communication terminal.The stationary information communication terminal illustrated in FIG.12E has at least a display portion 1401. The display portion 1401 can beprovided on a plane portion 1402. Further, operation buttons or the likecan be provided for the plane portion 1402. The stationary informationcommunication terminal illustrated in FIG. 12E can be used as anautomated teller machine or an information communication terminal (alsoreferred to as a multimedia station) for ordering information goods suchas a ticket (including a coupon). The liquid crystal display devicedescribed in the above embodiment can realize long intervals betweenwriting operations since a display period by one image-data writing islong. Therefore, by using the liquid crystal display device described inthe above embodiment for the stationary information communicationterminal illustrated in FIG. 12E, eyestrain can be suppressed even whenimages can be seen on the display portion for a long period of time, forexample.

FIG. 12F illustrates a display. The display illustrated in FIG. 12F hasa housing 1501, a display portion 1502, a speaker 1503, an LED lamp1504, operation buttons 1505, a connection terminal 1506, a sensor 1507,a microphone 1508, and a support base 1509. The liquid crystal displaydevice described in the above embodiment can realize long intervalsbetween writing operations since a display period by one image-datawriting is long. Therefore, by using the liquid crystal display devicedescribed in the above embodiment for the display illustrated in FIG.12F, eyestrain can be suppressed even when images can be seen on thedisplay portion for a long period of time, for example.

By applying the liquid crystal display device described in the aboveembodiment to display portions of electronic devices, multifunctionalelectronic devices can be provided.

This embodiment can be combined with any of other embodiments asappropriate.

This application is based on Japanese Patent Application serial No.2009-288283 filed with Japan Patent Office on Dec. 18, 2009, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A driving method of a liquid crystaldisplay device comprising a liquid crystal element, the driving methodcomprising: supplying data of an image signal to the liquid crystalelement in a first frame period; stopping supply of the data in secondto n-th frame periods, wherein n is a natural number larger than 1;inverting a polarity of a voltage, which is applied to the liquidcrystal element, between the n-th frame period and a (n+1)-th frameperiod; and compensating the voltage when an absolute value of thevoltage in the n-th frame period is different from an absolute value ofthe voltage in the (n+1)-th frame period, so that the absolute value ofthe voltage in the n-th frame period is the same as the absolute valueof the voltage in the (n+1)-th frame period, wherein the liquid crystalelement is electrically connected to a transistor which comprises anoxide semiconductor in a channel formation region, and wherein a stillimage is formed in the first to (n+1)th frame periods.
 2. The drivingmethod according to claim 1, wherein the liquid crystal elementcomprises a pixel electrode and a counter electrode with a liquidcrystal provided therebetween, and wherein the compensation of thevoltage is performed by changing a potential of the counter electrode.3. The driving method according to claim 1, wherein a carrierconcentration of the channel formation region is less than 1×10¹⁴ /cm³.4. The driving method according to claim 1, wherein the oxidesemiconductor comprises In, Ga, and Zn.
 5. The driving method accordingto claim 1, wherein the liquid crystal element is electrically connectedto a capacitor, and wherein the liquid crystal display device comprisesa node which is electrically connected to the transistor, the capacitor,and the liquid crystal element and which is directly connected only tothe transistor, the capacitor, and the liquid crystal element.
 6. Adriving method of a liquid crystal display device comprising a liquidcrystal element, the driving method comprising: counting number ofcontinuous frame periods which have the same data of an image signal,wherein supply of the data to the liquid crystal element is stoppedduring the continuous frame periods; inverting a polarity of a voltage,which is applied to the liquid crystal element, when the number exceedsa standard value; and compensating the voltage, so that an absolutevalue of the voltage is maintained after the inversion of the polarity,wherein the liquid crystal element is electrically connected to atransistor which comprises an oxide semiconductor in a channel formationregion.
 7. The driving method according to claim 6, wherein the liquidcrystal element comprises a pixel electrode and a counter electrode witha liquid crystal provided therebetween, and wherein the compensation ofthe voltage is performed by changing a potential of the counterelectrode.
 8. The driving method according to claim 6, wherein a carrierconcentration of the channel formation region is less than 1×10¹⁴ /cm³.9. The driving method according to claim 6, wherein the oxidesemiconductor comprises In, Ga, and Zn.
 10. The driving method accordingto claim 6, wherein the liquid crystal element is electrically connectedto a capacitor, and wherein the liquid crystal display device comprisesa node which is electrically connected to the transistor, the capacitor,and the liquid crystal element and which is directly connected only tothe transistor, the capacitor, and the liquid crystal element.
 11. Aliquid crystal display device comprising: a display panel comprising: aliquid crystal element; a transistor electrically connected to theliquid crystal element, the transistor comprising an oxide semiconductorin a channel formation region; and a driver circuit; and a displaycontrol circuit configured to supply an image signal to the drivercircuit in a first frame period and to stop supply of an image signal insecond to n-th frame periods, wherein n is a natural number larger than1, wherein the driver circuit is configured to invert polarity of avoltage, which is applied to the liquid crystal element, between then-th frame period and a (n+1)-th frame period, wherein the drivercircuit is further configured to compensate the voltage when an absolutevalue of the voltage in the n-th frame period is different from anabsolute value of the voltage in the (n+1)-th frame period, so that theabsolute value of the voltage in the n-th frame period is the same asthe absolute value of the voltage in the (n+1)-th frame period, whereina still image is formed in the first to (n+1)th frame periods.
 12. Theliquid crystal display device according to claim 11, further comprisinga memory circuit, wherein the memory circuit is configured to store dataof the first to n-th frame periods.
 13. The liquid crystal displaydevice according to claim 12, further comprising a comparison circuit,wherein the comparison circuit is configured to compare data of m-thframe period with data of (m+1)-th frame period, wherein m is a naturalnumber smaller than n, and wherein the comparison circuit is furtherconfigured to judge whether the data of the m-th frame period is thesame as the data of the (m+1)-th frame period.
 14. The liquid crystaldisplay device according to claim 13, further comprising a selectioncircuit, wherein the selection circuit is configured to stop supply ofthe image signal of the (m+1)-th frame period to the display controlcircuit when the comparison circuit judges the data of the (m+1)-thframe period as being the same as the data of the m-th frame period. 15.The liquid crystal display device according to claim 11, furthercomprising a counting circuit, wherein the counting circuit isconfigured to count number of continuous frame periods which are judgedas having the same data as the data of the first image signal.
 16. Theliquid crystal display device according to claim 11, further comprisinga compensation circuit, wherein the compensation circuit is configuredto compare the voltage applied to the liquid crystal element in the n-thframe period with that in the (n+1)-th frame period.
 17. The liquidcrystal display device according to claim 11, wherein a carrierconcentration of the channel formation region is less than 1×10¹⁴ /cm³.18. The liquid crystal display device according to claim 11, wherein theoxide semiconductor comprises In, Ga, and Zn.
 19. The liquid crystaldisplay device according to claim 11, further comprising a capacitor,wherein the liquid crystal display device comprises a node which iselectrically connected to the transistor, the capacitor, and the liquidcrystal element and which is directly connected only to the transistor,the capacitor, and the liquid crystal element.
 20. The liquid crystaldisplay device according to claim 11, wherein the display panelcomprises a display portion comprising the liquid crystal element andthe transistor, wherein the liquid crystal display device furthercomprises a memory circuit outside of the display panel, and wherein thememory circuit is configured to hold data of the image signal.